This invention relates to a fuel injection timing control method for internal combustion engines, and more particularly to a method of this kind wherein the fuel injection timing is determined in response to operating conditions of the engine.
In an internal combustion engine equipped with fuel injection valves, fuel mist injected into the intake pipe is carried by intake air into each cylinder via an intake valve. Therefore, generally, the injection timing of each fuel injection valve is determined such that fuel is supplied into the respective cylinder when the flow rate of the intake air becomes maximum, i.e. when the intake valve is opened, to thereby optimize atomization of fuel. This manner of determining the fuel injection timing is intended to compensate for a decrease in fuel atomization degree which occurs especially when the engine is operating in a low load condition wherein the intake air flow rate is relatively low, or when the engine is in a cold state, whereby the engine driveability is improved.
However, if this manner of determining the fuel injection timing of the fuel injection valves is applied to an engine of such design that the opening action of the intake valves are commenced before the respective exhaust valves are completely closed in order to improve the intake efficiency and thus to increase the engine output, there occurs a problem of increased concentration of HC in the exhaust gases, which is caused by the following phenomenon: for instance, when the engine is operating in a high load condition wherein the intake air flow rate is relatively high, the intake air flow rate rises so high immediately after the intake valve is opened that some of the injected fuel sucked into the cylinder is directly expelled from the cylinder through the exhaust valve without remaining there.
Therefore, it is necessary to set the fuel injection timing of the fuel injection valves to different timings depending on different operating conditions of the engine. Also, if the fuel injection timing is set to different timings in response to various operating conditions of the engine, it is necessary to change the fuel injection timing smoothly upon a transition in engine operating condition.
In order to meet these necessities, one possible way is to employ a timer for changing the fuel injection timing in response to changes in the engine operating condition. However, such way necessitates a complication of the construction of the fuel injection control system, and therefore impractical in terms of cost.